This invention relates generally to radio frequency circuits and more particularly to tunable radio frequency resonant circuits.
As is known in the art, tunable radio frequency resonant circuits such as tunable radio frequency filters are often used in radio frequency receivers to selectively transfer certain radio frequency (r.f.) signals therethrough. In particular, bandpass filters having a narrow frequency passband which may be tuned over a wide range of radio frequencies are often employed in r.f. receivers. Previously, such r.f. filters were provided by using an approach such as voltage tuned back biased diodes. Such an approach was inadequate for many receiver applications, for many reasons, but particularly as a result of its high insertion loss characteristic. A second approach, used in the prior art to overcome this insertion loss problem, is the use of magnetically tuned resonant circuits comprised of bodies of ferrimagnetic materials which, in the presence of a magnetic field, provides a resonant frequency circuit. A sphere of yttrium iron garnet (YIG) is often employed as the ferrimagnetic body. In prior art so-called YIG filters, for example, generally two coupling loops, one coupling loop disposed about an X axis and one coupling loop disposed about a Y axis are provided with a YIG sphere disposed within both loops. Generally, each coupling loop is a conductor shaped as a semicircle with each conductor loop being disposed around a different portion of the YIG sphere. This wire loop type YIG filter solved many of the problems associated with the prior art insertion loss characteristics of the voltage tuned back biased diodes. The principle of operation when using YIG as a resonant material is that in the presence of a suitably applied DC or steady magnetic field intensity H.sub.DC, a single crystal body of such material responds to an input r.f. signal, if the input signal has a frequency component substantially equal to the resonant frequency .omega..sub.o of the sphere. The resonant frequency (.omega..sub.o) of such a YIG sphere in a uniform resonance mode, is given as .omega..sub.o =.gamma.H.sub.DC, where .omega..sub.o is the centerband resonant radian frequency in the uniform resonance mode, .gamma. is a quantity which is a function of the material, and is generally referred to as the "gyromagnetic ratio", and H.sub.DC is the magnitude of the applied DC magnetic field. An r.f. signal fed to an input one of the aforementioned coupling loops, here the X axis loop is coupled through the YIG body, to the output one of the coupling loops, here the Y axis loop, if the frequency of such input r.f. signal equals the resonant frequency of the YIG circuit given by .omega..sub.o = .gamma.H.sub.DC. In operation in the uniform resonance mode, the external magnetic field H.sub.DC is applied in a direction along a Z axis aligning the spins of the electrons in the YIG sphere along the Z axis, and the input microwave frequency signal is fed to the input loop disposed about the X axis. In the presence of the external magnetic field, the resonant frequency energy fed to the input X axis loop is absorbed by the spins of the electrons in the YIG material making the electrons precess at the resonant frequency .omega..sub.o about the Z axis. In response to such precession, an RF magnetic moment is produced about the Y axis thereof, which induces a current in the Y axis coupling loop as described in an article entitled "Magnetically Tunable Microwave Filters Using Single Crystal Yttrium Iron Garnet Resonators" by Philip S. Carter, Transactions on Microwave Theory and Techniques, Volume 9, May 1961, pp. 252-260.
It has also been found that the resonant frequency of the above-described uniform resonance mode is a function of temperature for most orientations of the YIG sphere with respect to the external magnetic field H.sub.DC. However, along selected well-known orientations of the crystallographic structure of the sphere relative to the DC magnetic field, it is also well-known that the resonant frequency is substantially invariant with temperature variations. Generally, in the prior art, an initially orientated YIG sphere is disposed between the coupling loops and, in the presence of such loops, an iterative process is used where the resonant frequency of the filter is measured with the filter operating over the temperature range, and the sphere's final orientation is established when the variation in resonant frequency is a minimum over the temperature range. This multi-step process is a time consuming process since two alignment steps are required. It is thus a goal of YIG filter design to provide a YIG filter coupling structures having a controlled spatial relation to each other, and easy access to disposed therein a YIG sphere having a proper final orientation to minimize temperature variations in the resonant frequency of the output signal over the operating range of temperatures.
Further, an additional problem with the prior art YIG filter structure is that the r.f. magnetic field in the vicinity of the YIG sphere is generally not uniform. Because the r.f. magnetic field through the YIG sphere is not uniform even if the dc magnetic field H.sub.DC is uniform, the electrons in the YIG sphere will not oscillate in phase with each other, and the resulting phase differences encourage, in addition to the desired uniform resonance mode, undesirable higher order resonant modes of operation often referred to as "magnetostatic resonance modes" to occur. It is generally thought that these magnetostatic resonance modes result from nonuniform motion of the magnetization within the ferrimagnetic sample and resulting dipole interaction between the magnetic moments, due to the nonuniform distribution of the field throughout the YIG sphere. The strength of the magnetostatic resonance modes is dependent upon the shape of the resonant body, the distribution through the resonant body of the d.c. magnetic field, and the distribution of the r.f. magnetic field through the resonant body. Coupling in such modes permits the transfer of spurious energy signals which are outside the desired narrow passband of the resonant circuit. In general, the resonant frequency of the magnetostatic resonant modes differs from that of the uniform mode by an amount which is proportional to the saturation magnetization M.sub.o of the material comprising the sphere resonator, here the YIG sphere. Thus, the resonant frequency for all modes (the uniform mode as well as nonuniform modes) is given by .omega..sub.o =.gamma.(H.sub.DC +C4.pi.M.sub.o), where 4.pi.M.sub.o is the saturation magnetization and C is a constant that is different for different modes, and equals zero for the uniform mode.
In the prior art, magnetostatic resonance is often surpressed in YIG filters, for example, by the use of a dual stage filter with a first YIG sphere being a pure YIG crystal and a second YIG sphere being a doped YIG crystal. Gallium doping of a YIG crystal is often used to change the value of the saturation magnetization, and thus to change the nonuniform resonant frequency of the doped YIG sphere while in the presence of the same d.c. magnetic field H.sub.DC as the pure YIG sphere. The dual-stage filter is carefully designed such that each one of the YIG spheres will suppress the unwanted spurious energy produced by the other one of the YIG spheres. Further, certain applications where "steep skirt" (i.e. sharp cutoff frequency characteristics) filter response is required, additional stages are often used to provide the desired response. If, in order to surpress spurious energy a doped YIG crystal is used, the insertion loss of the filter is increased since doping of a YIG crystal, in general, provides a relatively lossy resonator in comparison to a pure YIG crystal. Further, where a single stage filter has a resonance characteristic which is adequate to provide the desired "steep skirt" filter characteristics, the use of a single stage filter is generally inadequate to suppress spurious energy transfer and thus a dual stage filter as described above is often employed. This is a costly approach in terms of increased circuit complexity and increased insertion loss and thus not a very desirable solution.
An additional problem in the art is the effect that a conductive surface, such as the coupling loops or an r.f. conductive housing of the filter, has on the resonance frequency of the YIG sphere. When a YIG sphere is located proximate to such a conductive surface, as in most prior art structures, there is a change in the resonant frequency of the YIG sphere. This change occurs because the proximity of the conductive surface to the YIG sphere distorts the r.f. magnetic field associated with the precessing spins of the electrons, and cause the magnetic field at the surface of the conductive surface to be in a direction parallel to the conductive surface. Normally, if the field was not distorted, the r.f. magnetic field in such cases would have components which are perpendicular and parallel to the conductive surface. This distortion of the r.f. magnetic field is caused by the high conductivity of the conductive surface and results in a shift in the resonant frequency. While at a selected temperature this "frequency shift" can be compensated for by changing the strength of the d.c. magnetic field H.sub.DC, this "frequency shift" is also temperature dependent making the compensation thereof more difficult over an extended temperature range. An additional problem occurs when, in response to the varying distorted r.f. magnetic field, a voltage is induced in the conductive surface and, in response thereto, eddy currents are produced. Since the conductive surface is not a perfect conductor, it has some dissipative characteristics and the eddy currents induced therein will dissipate power, resulting in the so-called "eddy current line broadening" effect. This so-called "eddy current line broadening" effect results in power dissipation, thereby increasing the insertion loss of the resonant circuit. In the prior art, eddy current line broadening is reduced by placing the YIG sphere further from the conductive surface, since the power dissipated in the conductive surface has previously been found to vary as 1/d.sup.4 where d is the distance between the conductive surface and the center of the YIG sphere. However, this solution generally results in reduced coupling efficiency and consequently unsatisfactory filter performance.